When you're applying for a job through the front or side door, in both cases you're responding to a position that has been advertised publicly.
I call this the "public" job market.
Most folks don't know this, but only 15% of jobs ever make it to the public job markets. So where do the others go? They're usually filled through the "private" job market. In other words, a candidate is nominated for the job before HR get involved, and the hiring process serves merely as a formality.
This is what the inside-door approach is all about.
The method is simple (though it does take some time to bear fruit):
First, you need to think about the kind of team you'd like to join. (If you've worked in a team like this in the past, that helps a lot. But it's not essential.)
Next, you do some desk research (mostly using LinkedIn), and you create a "target list" of managers who lead this kind of team.
Then, you reach out to these managers and invest in nurturing a professional "networking" relationship with them. Basically, you want them to view you as a resource they can turn to for advice, suggestions, or a sounding board.
Finally, you exercise patience and wait.
When a position opens up in their team, more often than not, one of these hiring managers will nominate you for the role. Provided you don't have any skeletons in the closet, the job is as good as yours.
(NOTE: I've seen managers actually create jobs in order to hire someone they like. I've also seen them refer people they like to managers in other companies with job openings.)
Here's what makes this approach powerful:
If enough managers know, like, and trust you – and if you've been strategic in how you built your "target list" – job openings like these come up all the time.
It's only a question of how many relationships you want to manage.
As you might expect, there's a right way and wrong way to reach out to managers and get them to know, like, and trust you.